Color image tube utilizing electroluminescent screen



Nov. 12, 1957 M. v. KALFAIAN 2,813,223

COLOR IMAGE TUBE UTILIZING ELECTROLUMINESCENT SCREEN Filed Jan; 11. 19551- I PHASE INVERTER I 7 1 iii (FL/p op) VIDEO F g.1 5 R .rm/ mwz ECUFRAME -BLANK 5m.

G 6 FILTER-STRIPES 9 & '8

,5 n: HOSPHOR .5

k ISPERSED l1 g 13 IELECTRIC 51 E m I Lu v k l U u E I, A 21 I aLAs sco/vouc TOR5 14 GREEN 15' RED T0 COLLECTOR AA/ODE IIVVEN TOR [IllZ,8l3,223 attented Nov. 12, 1957 COLOR IMAGE TUBE UTILIZING ELECTRO-LUMINESCENT SCREEN Meguer V. Kaifaian, Los Angeles, Qalif.

Application January 11, 1955, Serial No. 481,174

Claims. (Cl. 315-12) This invention relates to color television, andmore particularly to the provision of electroluminescent color screen inimage reproducing cathode ray devices. Its main object is to provide anelectroluminescent color screen, which may be activated by an electronbeam for the production of images in natural colors. Another object isto provide an electroluminescent color screen, the primary colors ofwhich may be selectively activated within the spot-area of a singleelectron beam, in either simultaneous or sequential additive systems,without altering the direction of the beam from its normal deflection offorming a scanning raster on the image screen. A corollary object is toprovide a composite screen which is capable of forming images indifferent primary colors withoutthe necessity of synchronous imagingprocess. Cathode ray. tubes for, reproducing televised images innaturalf'colors usually utilize some form of compositescreen,-comprising elemental primary-color component areas in the pathof the scanning beam. In conjunction with thesescreens a simultaneoussystem requires continuous control of the direction of approach of threeseparateelectron beams impinging upon the screen. In asequential'system, either the direction of approach or the normaldirection of a single beam is altered in a time sharing arrangementaccording to the arriving primarycolor signals. These systems in theirpresent forms require extremely accurate register-control adjustments,which are not desirable for practical purposes. In order to,obviatethese objections, a new type of color screen is provided herein, theprimaries of which may be selected simultaneously by a single scanningbeam without altering either its normal direction or direction ofapproach upon the image screen.

In ordinary cathode ray devices, luminescence is effected by an electronbeam bombarding the phosphorescent image surface; this action beingtermed as electron impact luminescence. Other methods ofproducingluminescence are also known and practiced, for example, by subjectingthe phosphorescent material to a fluctuating electric field; this actionbeing termed as electroluminescence. The device-utilized for the latterpractice comprises a phosphor dispersed thermoplastic dielectricmatrixplaced between two plane parallel conducting electrodes; one-of thelatter having light transparency. Whenan. alternating voltage isapplied'to these electrodes, the electric field produced between themcauses the phosphor cells to glow and emit light through thetranslucnfelectrode Sinceluminescence is produced by the electric fieldbetween two plane parallel electrodes, this type of structure is alsotermed as luminous capacitor. .The magnitude of luminescence emission:depends both ,uponfthefr equency and-amplitude of the applied voltage.As thefrequency of the applied voltage is increased, the lightbrilliance. is increased. However, the translucent'electrode, asutilized in its present form, is made'jof conductive glass, having highresistance electrically. .This high resistance increases the RC timeconordinary triode; causing the luminous capacitor to charge stant ofthe luminous capacitor, and prevents full charge and discharge at veryhigh frequencies, resulting in drop in output brilliance. To reduce thisresistive component in the structure, the present invention contemplatesto provide fine wire metallic screen in electrical contact with theconductive glass, whereby rendering the luminous capacitor operative athigh frequencies, and at he same time providing wide window area forviewing purposes. For further descriptive matter relating the luminouscapacitor, reference may be made to an article in Electrical Engineeringby Butler et al. on page 524, June 1954 entitled The ElectroluminescentLamp.

According to the brief description of the characteristic behavior of theluminous capacitor, as given above, the object of the present inventionis to provide a color image reproducing screen which comprises a mosaicof luminous capacitors having luminescence in different primary colors,and selectable simultaneously by a single scanning electron beam withouteither altering its normal direction of flow or the direction ofapproach upon the color screen. The function of the electroluminescentcolor image reproducing system will be better understood from thefollowing detailed specification when read in connection with theaccompanying drawings, wherein: Fig. 1 is a cross sectional view of theelectroluminescent image reproducing screen, including associatecomponent parts essential therefor; Fig. 2 is partly schematic andpartly diagrammatic illustration of the color image reproducingelectroluminescent screen, and circuitry associatedther'efor; and A, B,and C in Fig. 3 are details of component parts of the color screen.

Monochrome image In order to realize how the elemental luminouscapacimay be activated individually by the scanning beam, the systemwill be first described for monochrome image reproduction by way of theillustration in Fig. l, wherein, the target comprises a phosphordispersed dielectric matrix 1; a translucent conductor electrode 2; andsecondarily emissive electrodes 3, 4, etc. This target is disposedperpendicularly with respect to the normal flow of the beam 23, in avacuum envelope 22. The electron beam 23 is formed by a focusingelectron gun of conventional structure (such as used in television imagetubes), which comprises an electron emitter cathode 24; intensitycontrol electrode 25; and a focusing electrode 26. The scanning movementof the electron beam is achieved by the magnetic yoke 27 (ofconventional structure), which is energized by the output of thescanning wave generator block 28. The secondarily emitted electrons frommosaic elements 3, 4, etc., are collected by the electrode 5, which maybe in various physicalforms, for example, the graphite coating upon theinner surface of the funnel section of the envelope 22, as generallyreferred to ultor anode. Functionally, when the beam-accelerating fieldis so adjusted that the ratio of secondary current to the primarycurrent is greater than unity, the potential of bombarded electrode 3may be raised positive substantially equal to the potential of collectoranode 5, Where an equilibrium state is reached, and the net secondaryemission is then substantially unity. At this point, when the potentialof electrode 2 is varied by the video signal through resistance R, thepotential of electrode 3 will also vary through capacitive coupling, andeffect corresponding change in secondary emission, thus varying thecollector current. The effect, therefore, is the same as if theelectrode 2 were acting as the control grid, the electrode 3 as thecathode; and the collector 5 as the anode of an and discharge accordingto the applied video voltages. V/ith the assumption that the thicknessof dielectric sheet 1 18 less than the width of electrode 3, thetransverse distribution of electric field between the electrodes 2 and 3will be practically minimized, and the electroluminescence will beconstrained within this region; As will be noted, when the beam is movedupon electrode 4, the electroluminescence will accordingly move to thatregion. Thus, when the screen is made of a mosaic of minute electrodes3, 4, and a raster formed by the scanning beam, they will act asindividual triodes, and produce the desired luminescence according tothe applied image signals. Since luminescence is produced by changingelectric field, however, the stored charge in each luminous capacitormust discharge before the scanning beam arrives aft-that point; so as toefiect the required changing electric field. This may be achievedeither, first, by rendering the dielectric sheet slightly conductive toa degree Where the discharge time of each capacitor may be approximatelyequal to one frarne period, or second, by changing the polarity of thevideo signal by a phase inverting flip flop trigger circuit, which maybe operated by the frame blanking signals in a television system. Thelight emission from an elemental image area, in the latter case, will gothrough alow level dip during half of the frame period, and rise againat the end of the same frame, due to change of the electric field from astored state to zero state, andfinally resolve toa value representativeof the same image element in the succeedmg frame period. This conditionmay be attributed to the fact that difierent number ofelectroluminescent V crystallites will be activated, within an elementalimage "area, at each reversal of the potential across theelectroluminescent cell.

agglomerate will glow strongly when its physical shape has a definiterelationship with the electric field vector of theapplied voltage; thusmanifesting a rectifying action. Since however, in practical applicationthese crystallites will have random physical orientation in theelectroluminescent layer, it is accordingly evident that differentnumber of crystallites will glow during each reversal of the appliedvideo voltage. To increase the brilliancy of light emission, the innersurfaces of electrodes 3, 4, facing electrode 2, may be mademirror-like, for example, by electrodepositing the metal electrodes 3,4, on highly smooth surface of the dielectric sheet 1. As illustrated inFig. 1,'the fiip flop circuit is shown in block diagram 6, and thesource of video signals in block diagram 7. Since production of videosignals, including frame blanking signal, and the circuitry of variousforms of flip flop triggers are allknown and practiced in the art ofelectronics, no further description of these parts of the invention isdeemed necessary to be given herein.

Color image screen In reference to the above given specification, and byBriefly stated, each crystallite of an I of each of the other resistors.

to each stripe, and aligned therewith, is translucent conductor strip10. The conductor strips on stripes of each color are electricallyconnected to a corresponding resistor-e. g., those on green stripes toresistor R1, those on red stripes to resistor R2, and those on bluestripes to resistor R3. Next over the conductor strips 10 is a sheet ofthermoplastic dielectric matrix 11, dispersed with phosphorescentluminous cells. Finally, a mosaic of mutually insulated conductorelements, 12, cover the dielectric sheet in the region to be scanned bythe electron bean: 1 3. In this assembly, the resistors R1 to R3 areseparately energized by primary-color video signals arriving fromsources in blocks 14 to 16. Thus, when the high velocity electron beaml3 strikes one or several of the elemental electrodes 12, the secondaryemission establishes an electron path between these elemental electrodesand the electrode strips 10 through the resistors R1 to R3 and electroncollector anode (not shown for simplicity of drawing) in the manner asdescribed by way of the arrangement given in Fig. 1. The primarycolorvideo signals are thus varied between the three sets of electrode strips10 and the elemental electrodes 12 within the given area of electronbombardment. As described above by way of the arrangement given in Fig.1, the voltage phases of primary-color video signals. arriving fromsources 14 to 16 are reversed at the beginning of each succeeding frame;the phase reversing arrangements not being shown in Fig. 2, forsimplicity of drawmg.

Neutralization of capacitive coupling between adjacent strips In animage reproducing system of high image resolution, the number ofinterleaved strips should be substantially high. This means that theinherent capacitances Cd between adjacent strips will consequently behigh, as they will necessarily be in close proximity; especially whenthe number of strips is so high that more than one strip (for eachprimary color) occupy the beam area simultaneously. This tends to causecross fire between the signals representing diiferent primary colors,and accordingly, special compensating arrangement is utilized in theform as shown schematically in Fig. '2, wherein, the inherentcapacitances are indicated by Cd connected between the upper ends ofresistors R1, R2,

R3, and the neutralizing capacitors Cn connected between the upper endof one resistor, and the lower end Each of these resistors has a centertap '(electrically connected to the collector anode of secondaryemission, not shown in the drawing) ,rnaintained at group potential,-,sothat the video voltage appearing at the lower end of each of theresistors is opposite in polarity 'to that at its upper end, and whenelectroluminescent screen, as given in Fig. 1, is accord- L inglyconsidered as the basicnovel invention, and the color imaging processincluding associate apparatus thereof, are considered as novelimprovements thereupon. Thus, in accordance with the preferredembodiment of the present invention, the color image reproducing screenwill now be described by way of the illustration in Fig. 1, wherein, theelectroluminescent color screen is shown supported by atransparent faceplate 8,

which may serve as :the end wall of the image tube, through which theimage is viewed. A color-filtering film,j9, is, mounted over the innersurface of the face plate. This film contains a largenumber of verynarrow color selective stripes, corresponding to suitableprirnarycolors-e. g., greentG), red '(R) 'andblue (B); Next applied through acapacitance Cn adjusted to the proper value (approximately equal tocapacitance Cd), will balance out the eifect of the potential appliedthrough capacitor Cd. Conversely, althoug'h'part of the current from anyof the resistors passes through capacitance Cd toward the upper endterminal of the other resistors, a similar current passes-throughcapacitor'Cn toward the lower end terminals of said resistors; and thetwo currents tending to flow in opposite directions cancel one.

another, producing no net elfect on the three sets of conductor strips13. Thus each set of the strips 10 is energizedselectively[corresponding to primary-color signals arriving from sources14 to'16. j

, Target structure In reference tothe illustrative arrangement given inFig.1, for monochrome image reproduction, it was indicated that theelectrode 2 is electrically conductive, but visually translucent,whereby the electroluminescence of the target'may be emittedtherethrough for viewing purposes.

The translucent conductive electrode may be made by metal vaporization,few microns thick, on the surface of thermoplastic dielectricsheet 1.While this film of metal provides the required electrical properties ofthe electrode, the light transparency is equal approximately to 50% ofordinary glass plate. Another form of translucent conductor, that ispresently available, is the glass conductor, which is named nesa bytrade name. ThIS conductor has the desired light transparency, but itshigh electrical resistivity limits the operating etficiency of theelectroluminescent target at high frequencies. In order to lower theresistivity of this glass conductor, a fine wire metallic screen 21,such as shown at A in Fig. 3, may be laid in electrical contact over thesurface of the glass conductor coating. When the wire size of thismetallic screen is made very thin, the holes through which theluminescent image is formed will be wide enough for viewing, withoutobjectionable visibility of the metal screen. In the case of the colorscreen, as shown in Fig. 2, the translucent strips may be made ofconductorglass strips supported by thin solid metal wires, in the formas shown at B or C of Fig. 3. The conductor glass strips 17, at B, isshown edged with thin wire metal 18; and at C, the conductor glass strip19 is combined with a fine wire screen 20; either one of these formsbeing satisfactory for the purpose.

In constructing the electroluminescent image forming target, the thinwire screen may be first irnbedded on the inner surface of the glassface plate of the cathode ray tube, for example, by a known technique ofphotographing; etching; and filling the etched grooves of the glass, forexample, by electrodepositing the entire inner surface of the faceplate, or as an alternative, by first coating the entire inner surfaceof the face plate; photographing it with the proper design; and finallyetching the coated metal. The glass conductor may be coated either priorto, or after the metal screen is formed. The primarycolor stripes 9 maybe photographed over this structure. Finally, the thermoplasticdielectric sheet may be cemented over the photographed primary-colorstripes. The mosaic electrodes 12 may be mounted over the dielectricsheet either prior to, or after cementing over the primary-colorstripes. The mosaic electrodes may be formed in various ways, forexample, by first electro-deposition of a continuous metal film over thesmooth surface of the dielectric sheet; photographing a screen patternover this metal surface; and finally etching it into the rudimentalelectrodes. In this manner, the inner surfaces of these rudimentalelectrodes will have highly reflecting surfaces for increasing thebrilliance of the emitted luminous image.

Color filters, such as photographic color films, have the properties ofreducing the brilliancy of the transmitted light. Accordingly, thestriped color filter film 9 may be dispensed with, by constructing thethermoplastic dielectric sheet in narrow strips of phosphors having theproperties of luminescing in different primary colors.Electroluminescent phosphors having characteristics of luminescing indifferent colors have been known, for example, they may be zincsulfides, activated by combinations of lead, copper, chloride, andmanganese, and can be made in a range of colors from deep blue throughgreen to yellow and orange. By using a mixture, composed mainly of blueand yellow phosphors, white light of any desired color temperaturebetween 2,500 K. and 25,000 K. also may be produced. In the making ofthe electroluminescent layer, the desired phosphor powder may be mixed(by stirring) with the dielectric material, for example, Lucite in itsviscous state, and pressing between two smooth metal surfaces into asheet of about 100, or less, microns thick.

Various structural modifications may be made for improving the finalperformance of electroluminescent image formation. For example, in orderto reduce lateral distribution of the secondarily emitted electrons overthe mosaic electrodes, a barrier screen, or grid, may be placed infrontof the electoluminescent target, intercepting the primary electron beam.In various applications, barrier grids have been utilized and described,and as an example, one form may be referred to an article by Kazan etal. in RCA Review, December 1951, p. 703. As an illustration, however, abarrier grid may be structurally in the form of a-metal screen, such asshown at A in Fig. 3. A screen of this type may be utilized as acollector screen which may be placed between the primary electron beam,and the mosaic electrodes 3, 4, in Fig. 1. When the potential of thisis. adjusted equal to the final beam-accelerating potential, the primaryelectron beam will pass through the holes of the screen (part of thebeam current returning to the primary cathode through the screen byinterception), and hit the mosaic electrode 3, causing it to emitsecondary electrons. These secondary electrons will travel toward thescreen, where they will partly be collected by the screen, and partlypass therethrough; these latter electrons, however, eventually returningback to the screen, for collection; or partly traveling to the finalbeam-accelerating anode, which may be the aquaduct coating on the innerwall of the cathode ray image tube. With the utilization of a collectorscreen, the distance of secondarily emitted electrons from the mosaicelectrodes to the electron collector will be substantially uniformthroughout the image target, thereby substantially eliminating moireeffects on the image screen.

Other modifications may also be made, for example, due to the lowimpedance structure of the color screen, the video-voltages acrossresistors R1 to R3 may be de veloped through cathode followers. Withthese few examples in view, I wish it to be understood that numeroussubstitutions of parts, adaptations and modifications are possible andwill suggest themselves readily to the skilled in the art withoutdeparting from the spirit and scope of what is hereinabove set forth,and the limitations of this invention will accordingly be defined onlyby the appended I claims.

I claim:

1. An image reproducing system which comprises means for projecting anelectron beam; an image forming target in the path of said beam,comprising an electroluminescent phosphor layer disposed planeperpendicular to the normal direction of said electron beam a mosaic ofsecondarily emissive electrodes overlying the planar sur face of saidphosphor layer facing the beam, and a lighttransparent electrodeoverlying the planar surface of said phosphor layer remote from theelectron beam, whereby forming a mosaic of elemental capacitive elementsbetween the opposite surfaces of said phosphor layer; beam deflectionmeans and means therefor for forming a scanning raster upon said mosaicof electrodes with the beam; means for adjusting the velocity of saidbeam, whereby causing secondary emission from the scanned mosaicelectrodes; a collector anode for collecting last-named emission; animpedance means between said collector and the light-transparentelectrode, whereby forming secondary electron path between thelight-transparent electrode and the collector anode through saidimpedance means; a source of image display signals; and means forapplying these signals upon said impedance means, whereby energizingsaid mosaic of capacitive elements independently only at points of beamimpingement proportionally corresponding to the applied image signals,and thereby effecting image electroluminescence emerging through saidlight-transparent electrode.

2. The system as set forth in claim 1, wherein is in-.

cluded means for reversing the polarity of said image signals in saidimpedance means at return periods of said raster by said scanning beam,whereby effecting charge and discharge of said mosaic of capacitiveelements.

3. The system as set forth in claim 1, wherein said light-transparentelectrode is divided into a plurality of adjacently positionedlight-transparent strip-like electrodes, and electrically connected insequential steps into first, second and third sections; wherein saidphosphor layerc'Qmprises electroluminescentphosphors having thecharacteristics of exhibiting-luminescence in first, second an p m rycolors in stripe-like sections, adjacently aligned Withsaid strip-likesections; wherein said, im: pedance means comprises first, second andthird means, andmeans thereforfor terminatingsame, electrically commontosaid collector anode'and to said first, fi ondwand third strip-likesections, respectively; and wherein said source .of image signalscomprises first, second and thirdrsources' 'of image signals, and'means'thcre for for applying same 'upon said first, second and thirdimpedance means, respectively, whereby identifying theelectroluminescence upon said phosphor layer in difierent primary colorsrepresentative of said first,flse.cond and third image signals. 5 "j 4.The system as set forth in claim 1, wherein, said light-transparentelectrode comprises light-transparent conductive glass. V

V 5. The-system .as set forth i'nijclaimfl, wherein, said.

lightr-tran'sparcnt electrode comprises li ht-tran parent conductiveglass electrical-1y adjacent to'gifin'e Wirelow resistance metalliscreen, where y owcr n t e n ren s st component-:ofzsaidcondu vc g iam!atih i same m QHQ iJJEW op nsfc t ima formin elect um ncscence c pa sthec hrpueh-i: 4x1 1 References Cited file ofthisfatentt- 7 OT ERnnnnnmions Orthuber et 7 1; insanes ate. Image nithsifief," Opt. Soc.Am,., vol. 4'4, N o, 5;; April,19 54, pph297 to 299*,

